1. Field of the Invention
The present invention relates to a memory element including a semiconductor device and a signal processing circuit including the memory element.
2. Description of the Related Art
Transistors including amorphous silicon, polysilicon, microcrystalline silicon, or the like have been used for display devices such as liquid crystal displays conventionally. Nowadays, techniques in which such transistors are utilized for semiconductor integrated circuits are proposed (e.g., see Patent Document 1).
In recent years, a metal oxide having semiconductor characteristics, which is called oxide semiconductor, has attracted attention. The metal oxide is used for a variety of applications. For example, indium oxide is a well-known metal oxide and used as a material of a transparent electrode included in a liquid crystal display device or the like. Examples of metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, and zinc oxide. A transistor in which such a metal oxide having semiconductor characteristics is used for a channel formation region has been known (see Patent Documents 2 and 3).
Furthermore, a technique for reducing power consumption by shutting down power supply of a flip-flop circuit, which is enabled by saving data stored in the flip-flop circuit, in a memory element composed of a capacitor and a transistor including an oxide semiconductor, has been disclosed (see Patent Document 4). The saving is performed with the use of extremely small leakage current in an off state (off-state current) of the transistor including an oxide semiconductor.